Specific Process Knowledge/Etch/KOH Etch: Difference between revisions
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*Thermal SiO<math>_2</math> | *Thermal SiO<math>_2</math> | ||
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*~0.4 µm/min (60 <sup>o</sup>C) | *~0.4 µm/min (60 <sup>o</sup>C) | ||
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:in p<sup>++</sup> (doping level > 5x10<sup>19</sup> cm<sup>-3</sup>) | :in p<sup>++</sup> (doping level > 5x10<sup>19</sup> cm<sup>-3</sup>) | ||
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|'''Etch rate in SiO<math>_2</math>''' | |||
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:~7nm/min@80 <sup>o</sup>C | |||
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===Definition of structures=== | ===Definition of structures=== | ||
Revision as of 08:39, 17 November 2010
KOH etch - Anisotropic silicon etch
KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster.
KOH-etching is a highly versatile and cheap way to realize micro mechanical structures if you can live with the necessary SiFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _3} NFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _4} - or SiOFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} -masking materials and the potassium contamination of the surface. The latter necessitates in most cases a wet post-clean ('7-up' or RCA-clean) if the wafer is to be processed further.
At Danchip we use as a standard a 28 wt% KOH. The etch rate - and the selectivity towards a SiOFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} -mask - is depending on the temperature. We normally use T=80 oC but may choose to reduce this to e.g. 60 oC or 70 oC in case of a high-precision timed etch (e.g. defining a thin membrane). In some cases we recommend to saturate the standard 28 wt% KOH with IPA with an etch temperature at T=70 oC (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved compared to the standard etch. Key facts for the two solutions are resumed in the table:
KOH solutions
| 28 wt% KOH | 28 wt% KOH sat. with IPA | |
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| General description |
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| Possible masking materials |
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| Etch rate in Si |
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| Etch rate in SiOFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} |
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| Roughness |
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| Batch size |
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| Size of substrate |
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| Allowed materials |
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Definition of structures
Due to the almost inert (111)-planes it is possible by KOH etching to realize high aspect ratio structures in sigle crytalline silicon using the (111)-planes as sidewalls. In Si(100) these sidewalls are inclined - 54.7o with respect to the (100) surface - whereas in Si(110) the sidewalls are vertical (see figures below).
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For Si(100), the relation between the width of the bottom of the etched groove (Wb) and the width of the opening (Wo) at the wafer surface in a groove etched to the depth l is given by:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle W_b = W_o - 2lcot(54.7^o) = W_o - \sqrt{2} l}
Definition of <110> alignment structures
The etch rate dependence on the crystallographic planes can be used to determine the <110> crystal directions with high precision (better than +/- 0.05 o). A fast method for doing this, using the symmetric under-etching behavior around but not at the <110>-directions, was described by Vangbo and Bäcklund in J. Micromech. Microeng.6 (1996), 279-284. High-precision control of the <110>-direction during alignment can be necessary in order to control the dimensions of KOH-etched structures (e.g. precise control of V-groove dimensions). A dedicated mask (MASK NAME) has been designed for this purpose.
Etch rates: Empirical formula (Seidl et al)
The following empirical formula can be used for concentrations in the range of 10-60 wt%:
R = k0 [H2O]4 [KOH]0.25 e-Ea/kT,
where k0 = 2480 µm/hr (mol/l)-4.25, Ea = 0.595 eV for Si(100)
and k0 = 4500 µm/hr (mol/l)-4.25, Ea = 0.60 eV for Si(110)